That is a great idea! Could you suggest it as a new idea?
For now, you could do this as follows:

1. Create an un-pitched percussion part.
2. Write a measure with a hit on beats 1 and 4
3. Select measure and type R to repeat many times
4. In the instruments panel, un-select that part from view so it is hidden

I don't know if this is already an option, I haven't seen it, but it would also be really helpful if you could adjust which beats it clicks on. For example, if you are in 6/8 a good option to have would be clicks just on the 1 and 4 counts instead of all 6.

If Noteflight had a metronome, I would switch to Noteflight like a shot as it works on so many platforms. Most of my work involves writing tutorials for beginning wind instrumentalists. A metronome or click allows the beginner to hear two empty bars at the beginning before beginning to play, and the metronome keeps the beginner in time and inculcates a good sense of time. The use of Noteflight promulgates the idea in the beginners mind that this is the notation program of choice, with the likelihood that he/she will become a future Noteflight user.

I'd like to add my support to this idea. I'd also suggest there be an option to start the click track one measure before the music, it would be quite beneficial for people using it to practice vocal parts

My son has the hardest time with maintaining a consistent tempo, playing along with noteflight helps, but it would be great for him to have the metronome clicking in his head whilst he is playing and even after!

My son plays violin and we use noteflight to enter difficult passages in the music so he can play along. It's great that we can set the tempo but it would be really useful to also have a metronome "click" on the beat. A simple ON/OFF control would be great.